1. Field of the Invention
The present invention relates to an image signal processing apparatus and method, and specifically relates to an image signal processing apparatus and method which is able to prevent generation of a fog phenomenon (the phenomenon that the background of an original is recorded lightly or thinly, i.e., "dirty", or yellowed) in a digital reproducing apparatus, etc.
2. Related Art
In a conventional digital reproducing apparatus, to prevent generation of a fog phenomenon, there has been proposed a method in which one converts output density in accordance with image information obtained by scanning originals as shown in FIG. 13.
For example, when an input image signal which is obtained by detecting an image to be processed and which corresponds to the image density thereof is converted to an output image signal which corresponds to an output density level being suitable for subsequent processing, the input image signal under a predetermined level X is converted to a predetermined constant output level, and then insofar as the input image signal is over the level X, the higher the level of input image signal, the higher the level of output image signal, in an almost linear relationship. This method is proposed in Japanese Laid Open Patent No. 1981-165462.
As another example, the density of an original is detected, a histogram is formed in accordance with densities of many points in the original, and the input signal is converted to a predetermined output signal in accordance with a pattern of the histogram. This method is proposed in Japanese Laid Open Patent No. 1982-45564.
However, these conventional methods which use density conversions to prevent generation of the fog phenomenon, cannot reproduce a part of low density, e.g., skin of human face, clearly or with sufficient fidelity.